This invention relates generally to a method and apparatus for filtering periodic and aperiodic noise from a signal having a data component and a noise component. More particularly, this invention relates to a technique of cancelling noise which interferes or otherwise disturbs measurement-while-drilling (MWD) signals obtained during the drilling of subterranean wells.
The mud column in a rotary drill string may serve as the transmission medium for carrying signals of downhole parameters to the surface. This signal transmission is accomplished by the well known technique of mud pulse generation whereby pressure pulses are generated in the mud column which are representative of sensed parameters down the well. The drilling parameters are sensed in a sensor unit in a bottom hole assembly (BHA) near or adjacent to the drill bit. Pressure pulses are established in the mud stream within the drill string, and these pressure pulses are received by a pressure transducer and then transmitted to a signal receiving unit which may record, display and/or perform computations on the signals to provide information on various conditions down the well. The mud pulses may be generated by any of the known measurement-while-drilling (MWD) systems such as disclosed in U.S. Pat. Nos. 3,982,431, 4,013,445 and 4,021,774, all of which are assigned to Teleco Oilfield Services, Inc. of Meriden, Connecticut (assignee of the present invention).
The average pressure measured in the mud column of the drill string or standpipe is known as standpipe pressure or SPP. As a result of the drilling there are many energy sources that disturb the average pressure measured in the standpipe. One such energy source is of course, the signal from the MWD tool itself. As mentioned, this is a pressure modulated digitally encoded signal which communicates information from sensors located near the bit. Unfortunately, other energy sources cause disturbances (e.g. noise) that interfere with the MWD signal. Examples of these are fluctuations caused by the action of the mud pumps, bit bounce and rapid vibration motion of the drill string. These disturbances cause pressure changes that confound the signal from the MWD tool so that the reliability of the decoded MWD information is reduced.
Bit bounce and the rapid motion of the drill string can be related to the dynamic variations in the drilling process itself. One primary source of these vibrations has been identified with the nature of the interaction between the bit and the formation. In SPE PAPER 16660 entitled "The Effects of Quasi-Random Drill Bit Vibrations Upon Drill String Dynamic Behavior" (Sept. 1987), the author states "One of the main sources of drill string vibrations is the interaction between the drill bit and the formation. Downhole measurements of forces and accelerations within the bottomhole assembly have shown that the vibrations at the bit have large quasi-random components, both for axial and rotational movements. These quasi-random vibrations are probably due to uneveness of the formation strength, random breakage of rock, and amplification of these effects by mode coupling . . . ".
It has now been discovered by the inventors herein that the effect of these vibrations are the main cause of offending pressure variations seen in the standpipe and that indirect measurements at the surface can be used to reduce the effect of the offensive pressure variations. One notable pressure disturbance (i.e. source of noise) is a result of axial and torsional vibration that often manifests itself, for example, as a stick/slip action of the bottomhole assembly (BHA). Indicators of these downhole vibrations can be monitored at or near the surface.
This stick/slip phenomenon is described in a paper entitled "A Study of Slip-Stick Motion of the Bit" by A. Kyllingstad and G. W. Halsey, Society of Petroleum Engineers (SPE) Paper 16659, Sept. 1987. As discussed in that paper, torsional oscillations are caused by alternating slipping and sticking of the bottom hole assembly (BHA) as it rotates in the borehole. This phenomenon is associated with a large amplitude, sinusoidial and often saw-tooth like variation in the applied torque. The term slip-stick motion refers to the belief that the amplitude of the torsional oscillations becomes so large that the drillcollar section periodically comes to a complete stop and does not come free until enough torque is built up in the drill string to overcome the static friction.
By observing these phenomena the inventors herein have discovered the following features which result from downhole vibrations such as the stick/slip action of the BHA:
(1) When the vibrations such as the stick-slip action at the bit occurs, hydraulic pressure pulses in the drilling fluid are created that travel to the surface and can be detected in the standpipe.
(2) When the vibrations such as the stick-slip action at the bit occurs, input drive torque rotating the drill string changes and these changes have a relationship to the pulses detected in the standpipe.
(3) The shape and timing of the drive torque measurement is different from the pressure pulses detected in the standpipe. This is because the reflection of the bit torque travels in the steel of the drillpipe while the pressure signal travels in the drilling mud and the channel phase velocities and dispersive effects are quite different. But, if the measurement of the surface signal such as the input drive torque is modified in shape and timing, it can be made to approximate the pulses detected in the standpipe.
It will be readily appreciated that the pressure disturbance created when the stick/slip action occurs causes disruptive noise which is detrimental to the integrity of the MWD signal. Noise cancellation techniques are known which are usually effective for noise reduction and signal to noise ratio (SNR) enhancement. These known techniques include adaptive filters such as the least mean-square (LMS) and the recursive least square (RLS); and are effective when (1) noise reference is available, (2) the noise is periodic, (3) the noise is uncorrelated with the signal to be enhanced; and (4) the noise statistics are changing slowly. Another known noise cancellation technique for periodic and slowly changing noise is disclosed in U.S. Pat. No. 4,642,800.
In the above discussion, bit torque reflected to the surface is measured by monitoring the drive torque to the drill string. Measurement of the torsional accelerations at the surface (at or below the Kelly) will also produce the desired results. If this technique is employed, it would be advantageous to measure the axial accelerations as well. Axial vibrations at the surface are indicative of downhole axial vibrations such as bit bounce which is another source of hydraulic pressure pulses. These pulses also are detectable in the standpipe and hinder the accuracy of pressure pulse MWD data reception. These measurements of surface axial vibration would be treated in a similar manner to cancel the pressure pulse effects of downhole axial vibration. In fact, the stick-slip action is a combination of torsional and axial movements, both of which are reflected in the drive torque. By measuring torsional and axial motion separately, such as can be done with accelerometers, the two components can be separated and treated individually.